1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and to a semiconductor device; in particular, it relates to a method of manufacturing a semiconductor device and a semiconductor device that can regulate the stress of a silicon nitride film which is used as a final protective film on wiring to suppress warping and deformation of wafers caused by distortion of specially manufactured products such as high-voltage products and MEMS (Micro Electro Mechanical Systems).
2. Description of Related Art
Final protective films (hereinafter sometimes referred to as “passivation films”) used in current semiconductor products are, in the mainstream, plasma nitride films (hereinafter sometimes referred to as “P—SiN films”) reaction-formed by the plasma CVD method with a gas mixture of SiH4, NH3 and N2. When a standard P—SiN film is formed on a silicon (Si) substrate, compressive stress is observed, and depending on the conditions of formation, the stress can be controlled to within a range of about −50 MPa to −500 MPa (where the minus sign indicates the direction of compression). Further, this feature is not seen with other generally-used CVD methods (such as LP-CVD or normal pressure CVD methods) and, therefore, it is an extremely versatile process.
Recently, there has been demand for further miniaturization of devices and, together with this, a demand for passivation films having various characteristics. These characteristics concern the resistance of the passivation film, after formation thereof, to heat stress and mechanical stress in post-processing, as well as its ability to act as a barrier to the external atmosphere, with particular regard to humidity and the like. Moreover, together with the miniaturization of semiconductor devices and increases in multilayer wiring, electromigration and stress migration, in particular, have been focused on as the cause of faults which are known to affect the internal stress of the passivation film itself.
A technique in which a passivation film is formed as a single layer having low tensile stress in order to suppress the above migrations is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 6-333922.
A multiple layer technique in which a passivation film has a two layer structure manufactured by a high density plasma CVD method, in which a film having low compressive stress is laminated on a film having high compressive stress, is disclosed in JP-A No. 5-6890. Further, a single layer passivation film in which a surface layer portion and a lower layer portion have different film qualities, and the lower layer portion has a low compressive stress, and the surface layer portion has excellent humidity resistance, is disclosed in JP-A No. 6-291114.
When forming the standard P—SiN film described above, the film is formed on a semiconductor substrate by high temperature heating at about 400° C. At this time, the semiconductor substrate (silicon substrate) expands in accordance with its inherent thermal expansion rate (<0.0003%/° C.). The P—SiN film formed on the semiconductor substrate does not exhibit stress at the time of forming and is thus stress-free with respect to the substrate. However, after the P—SiN film has been formed, the semiconductor substrate returns to a normal temperature and, due to the difference in thermal expansion rates between the semiconductor substrate and the P—SiN film, that is, due to the difference in respective contraction amounts thereof, the entire semiconductor substrate deforms and, usually, the P—SiN film contracts less than the semiconductor substrate after being formed, and the warping in the positive direction increases. Thus, since the thermal expansion rate of the P—SiN film is less than that of the semiconductor substrate, although it may be possible to reduce the warp amount in the positive direction by controlling the stress in the direction of compression, it is difficult to reverse the warp direction to a negative direction.
The formation and alleviation of warping of the semiconductor substrate using the passivation film described above is shown in FIG. 6A and FIG. 6B. In FIG. 6A, when a standard passivation film 30 is formed on a semiconductor substrate 10 that does not exhibit warping, warping occurs in the semiconductor substrate due to the differences in the thermal expansion (contraction) rates between the two. Further, in FIG. 6B, a semiconductor substrate 10 has in its initial state a convex warp toward an upper part thereof (hereinafter sometimes referred to as “positive direction warping”), and when a passivation film 40 having compression stress is deposited thereon, the warping of the semiconductor substrate is alleviated. However, as stated previously, since a standard P—SiN film has compressive stress, when a standard P—SiN film is formed on a semiconductor substrate having, in its initial state, a concave warp toward a lower part thereof (hereinafter sometimes referred to as “negative direction warping”) it is difficult to alleviate the warping of the semiconductor substrate, since warping in a negative direction is promoted.
Generally, it is known that the humidity resistance of the passivation film improves when the internal stress of the film is a high compressive stress.
Thus, since the passivation film disclosed in JP-A No. 6-333922 is a single layer passivation film having tensile stress as internal stress, the humidity resistance thereof is poor. Further, even though stress is exhibited in a direction that alleviates warping in a negative direction, this stress is weak, and it is difficult to sufficiently alleviate warping in a negative direction. Moreover, even though the film can provide tensile stress, alleviating a concave warp toward a lower part of the semiconductor substrate is problematic.
Although the passivation film of JP-A No. 5-6890 has a laminated structure, since the internal stress of each of the film layers is compressive stress, it is difficult to alleviate warping of a semiconductor substrate in a negative direction. Further, because the film is formed by the high-density plasma CVD method, although the film quality is high, the manufacturing process is laborious, and thus it is impossible to avoid using a manufacturing process that differs greatly from conventional manufacturing processes.
Although the passivation film disclosed in JP-A No. 6-291114 has good humidity resistance at a surface portion thereof, each of the surface portion and the lower portion have compressive stress and, therefore, the warping of the semiconductor substrate in a negative direction is promoted, and it is difficult to alleviate the warping of the semiconductor substrate.